Because of the way THC is metabolized plus differing tolerances among different users the only way to find out if you're too high to drive would be to test for actual impairment of skills like reflex reaction times, etc.

I bet if they did this a lot of people who aren't high would also fail the test due to legal prescription drugs, stress, illness and a bunch of other causes like just plain old not having their shit together.

If THC were a big problem with drivers we'd already know it. The scale of the costs of driving while under the influence of alcohol were (and are) obvious from the scale of the carnage. Where is the rash of pot related accidents among millions of smokers over the last few decades? Show me any valid study that demonstrates a danger greater than cell phones and many other distractions we take for granted.

Yes, one could make a pretty valid argument that people shouldn't drive stoned. And I could probably build a pretty good counter that the real problem with driving is aggression, and maybe pot would chill people out enough to actually drive safer. The truth is there are a lot of people who probably shouldn't be allowed to drive at all.